- add `netif_get_ipv6()` to query IPv6 (as a slim convenience wrapper
for `netif_get_opt()`)
- add `netifs_get_ipv6()` to query IPv6 addresses of all netifs
- add `netif_print_ipv6()` to print the IPv6 address(es) of a single
netif
- add `netifs_print_ipv6()` to print the IPv6 address(es) of all netifs